This invention relates to a piston, particularly for internal combustion engines, which piston comprises a top part including at least a piston head and a ring-carrying portion, and also comprises a bottom part, which is connected to the top part by suitable means.
The fuel consumption of modern internal combustion engines can be decreased by a reduction of the weight of components of said engines and a contribution to the saving of energy can thus be made. Internal combustion engines are thus required to be as light in weight as possible and should also meet high comfort requirements as regards the production of noise and vibrations. Said targets of development can be reached in part by the use of pistons which are light in weight so that they have small oscillating masses. For this reason persons skilled in the art face the problem of designing light-weight pistons which are suitable for use in modern internal combustion engines, but where a reduction in weight does not reduce the reliability in operation and the life of the pistion.
Pistons intended for use, inter alia, in internal combustion engines have been designed for a saving in weight and a reduction of friction losses in that the skirt has been provided with recesses adjacent to the horizontal plane containing the piston pins and in that the hubs of the piston pin bosses have been arranged as close as possible to the center of the piston. The distance between the outor ends of the hubs of the piston pin bosses is only 60 to 75% of the diameter of the piston so that a very short piston pin can be used, which has an overall length of up to 60% of the diameter of the piston. The lower portion of the skirt of the piston is usually circular, although the recesses may alternatively extend as far as to the lower end of the skirt so that the circular skirt portions below the piston pin are entirely eliminated.
It is also desirable to minimize the distance between the axis of the piston pin and the top of the piston head, i.e., the compression height. This will not only result in a lighter weight and in lower friction losses, but will also permit a reduction of the overall height of the crankcase. For this purpose it is known from Published German Application No. 32 35 220 to provide, for internal combustion engines, a piston which comprises an outer piston and the piston skirt, and an inner, second piston part, which carries the hubs of the piston pin bosses. To permit the use of long connecting rods and in order to reduce the tilting moments generated by the mass forces of the piston, the second part of the piston is firmly connected to the first part of the piston and the axis of the piston pin extends substantially through the center of gravity of the entire piston approximately on the level of the ring-carrying portion. Such a piston has the decisive disadvantage that the inner part of the piston, including the hubs of the piston pin bosses, is directly secured to the piston head so that the piston pin is strongly heated by the piston head, which is at a high temperature of up to 280.degree. C. As a result, an adequate lubrication cannot be ensured because the lubricant is coked; this results in a heavy wear of the piston pin and of the hubs of the piston pin bosses.